C.P.S. Larsen

The reconstruction of boreal forest fire history from lake sediments: A comparison of charcoal, pollen, sedimentological, and geochemical indices

Abstract Fossil charcoal, fossil pollen, sedimentological and geochemical analyses of lake sediments have been used previously to reconstruct a history of local fires and resulting vegetation change. The rationale behind these approaches is described and the usefulness of each technique for reconstructing fire history in the boreal forest is assessed empirically. Historical and dendrochronological records provide regional and local fire histories for a site in Wood Buffalo National Park, Alberta, Canada. The local and regional history of fires is compared with the microscopic charcoal content, macroscopic charcoal content, elemental carbon content, fossil pollen content, sedimentology, and geochemistry of annually laminated sediments from a small lake. There is no significant correlation between the abundance of microscopic charcoal, macroscopic charcoal and total elemental carbon content of the sediments. Automated measures of microscopic charcoal abundance made with an image analysis system are correlated significantly with optical counts of microscopic charcoal. None of the charcoal measures provide unequivocal records of local fire activity and the abundance of microscopic charcoal appears to be influenced by variations in regional fire activity. However, the highest depositional rate of macrofossil charcoal occurred during the time of a fire that burned within the drainage basin. Variations in sedimentological measures and geochemistry do not correlate with local fire activity. Fossil pollen percentages and accumulation rates display a pattern of variation that is consistent with observed vegetation responses to fire in the boreal forest. What is likely apparent in the pollen record are the results of a series of burns of sufficient extent and intensity to kill most of the above-ground biomass of the vegetation in an area at least as great as the drainage basin. The inability of evidence from lake sediments to provide detailed histories of past fire activity is not surprising given the wide range of variation in the spatial extent, proximity, intensity and impact of individual fires.

Climatically Induced Change in Fire Frequency in the Southern Canadian Rockies

The purpose of this study was to partition the components of a mixed fire frequency, and empirically relate these components to temporal and spatial differences in fire frequency. The method of reconstructing the fire frequency was to build a stand-origin map, from this estimate the mixed time-since-fire distribution, and then use a graphic technique to partition the mixed distribution into two homogeneous fire-frequency distri- butions. The fire frequency for the last 380 yr in the 495-km2 Kananaskis Watershed showed a temporal change in fire frequency at - 1730. This change in fire frequency is related to a change from a warmer and drier climate before 1730 to a cooler and moister climate since then. When the fire-frequency data were partitioned by a graphic technique, the two resulting fire frequencies fit negative exponential distributions. For the period 1730-1980 the fire cycle (the time required to burn an area equal to the area of study, or one divided by the scale parameter of the exponential distribution) was 90 yr. For the period before 1730 the fire cycle was 50 yr. The Watershed could not be subdivided into smaller, spatially ho- mogeneous fire-frequency units. The negative exponential distribution of fire frequencies suggests a constant hazard function (mortality force). This constant hazard was corroborated by a spatial correlation (Morans I) test which found no spatial pattern other than ones expected by chance between forest ages on either side of all fire boundaries. Thus, there was no tendency for young stands to be associated either with only younger- or only older- aged stands. Other studies on fire behavior in the Watershed corroborate the short fire cycle, constant hazard, and lack of spatial fire-frequency differences, and suggest thai the regional climate control of the temporal fire frequency is related to a characteristic synoptic weather pattern and the resulting high intensity and high rate-of-spread of fires.

Lake morphometry, sediment mixing and the selection of sites for fine resolution palaeoecological studies

Abstract Palaeoecologists are increasingly using close interval or continuous sampling of lake sediments to construct fine resolution records of environmental change. The degree of temporal resolution sought in these studies is typically decades or years. The use of lake sediments for such fine resolution reconstructions assumes that temporal mixing of the sediments is negligible. In this paper we examine the physical and biological processes that cause sediment mixing in small lakes. We use physical models and empirical data to develop a set of heuristics relating lake depth and area to sediment mixing by slumping, waves, currents and bioturbation. We also develop a heuristic that allows information on lake depth and area to be used to determine if the sediments of a lake will likely be massive or annually laminated. In general, the degree of sediment mixing likely to occur in a small lake is directly related to the depth and surface area of the lake. However, each of the agents of mixing has a different critical relationship with lake depth and surface area. We present a series of equations and a summarizing figure that, based on lake surface area and maximum depth, allows the type of thermal stratification (polymictic, dimictic or meromictic) and the degree of sediment mixing present in the lake to be inferred.

Reconstruction of Fire Disturbance and Forest Succession from Fossil Pollen in Lake Sediments: Potential and Limitations

The size of a fire, its proximity to a lake and the size of the lake all affect the way the fire is represented in pollen records. To quantify how these three factors affect pollen representation of disturbance events such as fires, computer simulation models of pollen deposition on a lake were applied. As a model input, we used both a vegetation map of the boreal forest around Rainbow Lake A (3 ha in size), Alberta, Canada, and an hypothetical map of patchy vegetation, which consists of circular patches of aspen and spruce in a matrix of pine, with a circular lake (3 ha) at the center.

AN 840‐YEAR RECORD OF FIRE AND VEGETATION IN A BOREAL WHITE SPRUCE FOREST

Boreal forest fire and vegetation history was examined using an 840-yr fossil pollen and charcoal record from a site dominated by Picea glauca in Alberta, Canada. The record was created using contiguous 5-yr samples of annually laminated sediments from a 2.7-ha lake (Rainbow Lake A in Wood Buffalo National Park). Following a peak in the microscopic charcoal accumulation rate (CHAR), there is a sequence of peaks in sediment thickness and in 14 of the 21 common pollen and spore taxa. The general pollen sequence is an initial peak in herbs, then shrubs, then deciduous trees, and finally conifers and Sphagnum. The peak value for each taxon differs following individual fires, suggesting that a site does not undergo the same postfire vegetation sequence following all fires. Based on the CHAR, pollen, and sediment thickness records, at least 12 large, local fires are apparent. The average time interval between the 12 fires is 69 yr. Significant periodicities in CHAR and in 11 of the pollen taxa range between 95 ...

Relations between lake morphometry and the presence of laminated lake sediments: a re-examination of Larsen and Macdonald (1993)

Abstract In this paper we conduct an empirical test of a published equation which relates lake surface area and maximum lake depth to the presence or absence of laminated lake sediments. A 297 lake dataset from New York State and six regions in Canada, representing a number of biogeoclimatic zones, is employed. The results suggest that deeper lakes are more likely to contain laminated lake sediments than are shallower lakes. The percentage of lakes incorrectly predicted to contain laminated sediments (false positives) and that incorrectly predicted to contain massive sediments (false negatives), was much higher than that found in the study in which the original equation was developed. Its low predictive ability suggests, therefore, that in addition to lake morphometry, many other factors affect the formation and preservation of laminated sediments.